The invention relates to a method for actuating at least one capacitive actuator element, in particular a fuel injection valve of an internal combustion engine. The invention also relates to a device for carrying out this method.
When a capacitive actuator element is actuated by means of resonant output stagesxe2x80x94the capacitances of the charging capacitors and of the actuator element form an oscillatory circuit with the inductance of the recharging coilxe2x80x94the power components which form the oscillatory circuit and their tolerances have a large influence on the charging and discharging times of the actuator element, which leads to effects on the fuel injection quantity in the known fuel injection valves.
From the point of view of combustion technologyxe2x80x94and in order to obtain the smallest possible fuel injection quantitiesxe2x80x94charging times which are as short as possible are to be aimed at but these lead to high noise emissions. For this reason, the charging times are regulated and short charging times are predominantly used in those operating ranges in which the noise emissions caused by them are not felt to be disruptive, for example at high engine speeds at which their sound is drowned out.
In the non-prepublished DE 199 44 734.9, in which the charging capacitor is configured for a maximum charging period, it is proposed to switch over the oscillatory circuit to a freewheeling circuit in order to shorten the charging time.
DE 196 32 872 A1 discloses a method and a device for actuating at least one capacitive actuator element which can be charged to different setpoint voltage values, which brings about a variable charging time which can be shortened by means of premature switching off, starting from a maximum charging time predefined by capacitances and inductances.
DE 196 52 801 C1 discloses a method and a device for actuating at least one capacitive actuator element which is charged with a constant amount of energy which, despite the variable actuator element capacitance which is dependent on temperature is kept constant by varying the charging voltage.
DE 198 05 184 A1 discloses a method and a device for determining a temperature of a piezoelectric actuator element, the actuator element being charged in a clocked fashion from an energy store which is supplied with energy from a voltage transformer and is also discharged again into the energy store in a clocked fashion.
The object of the invention is to specify a method for actuating a capacitive actuator element of a fuel injection valve of an internal combustion engine by means of which a variable charging time of the actuator element can be obtained. The object of the invention is also to provide a device for carrying out this method.
This object is achieved according to the invention in terms of a method for actuating at least one capacitive actuator element by means of a charging oscillatory circuit which contains the actuator element, wherein
in order to bring about a variable charging time, the capacitance of the charging source is dimensioned in such a way that the resonance frequency of the charging oscillatory circuit gives rise to a predetermined, average charging time,
in order to bring about a minimum charging time up to the average charging time, the flow current is switched over from the charging oscillatory circuit to a freewheeling circuit at a specific time, and
in order to bring about the average charging time up to the maximum charging time, the flow of current is switched over from the free-wheeling circuit back to the charging oscillatory circuit at a specific time.
Furthermore, the object is achieved by a device for controlling at least one capacitive actuator having a series circuit composed of a charge source composed of a charging capacitor which can be charged from a power source, and a recharging capacitor, a charging switch, a first diode, a recharging coil and the actuator element, and having a discharging switch which connects the connection point for the first diode and recharging capacitor to ground, and a controlled circuit which controls the charging and the discharging switches via a second diode, wherein a series circuit of a third diode and of a freewheeling capacitor leads from the connecting point between the second diode and discharging switch to the connecting point between the recharging capacitor and recharging coil.
The discharging switch may be bypassed by a diode which conducts current in the direction of the second diode. The device may further comprise a series circuit composed of fourth and fifth diodes, which conduct current in the direction of their connecting point, that is connected in parallel with the freewheeling capacitor. Furthermore, a discharging switch which is switched on and off in synchronism with the discharging switch may be arranged between the connecting point of the fourth and fifth diodes and the charging capacitor.
The method according to the invention consists in the fact that, in order to achieve variable charging times, the current flowing in the primary resonance circuit during the charging operation of the capacitive actuator element is redirected into a freewheeling circuit in order to variable the charging time, as a result of which the charging operation is significantly shortened. In order to prolong the charging time, firstly the current flowing in the primary resonant circuit is redirected into the freewheeling circuit and then the primary charging circuit is switched on again as long as current is still flowing in the freewheeling circuit. In this way, the charging time can be divided into two or more phases in which the primary oscillator circuit and the freewheeling circuit are alternatively switched on, as a result of which the charging time is prolonged.
The device according to the invention also has the advantage of making possible protection against short circuiting and personal protection. This is significant as the output of the actuator circuit can be connected directly to high intermediate voltages, for example 80V, and is connected to the cable set of the internal combustion engine in which a short circuit cannot be ruled out.
An exemplary embodiment of a device for carrying out the method according to the invention is explained in more detail below with reference to the schematic drawing, in which:
FIG. 1 shows the circuit of an exemplary embodiment, and
FIG. 2 shows a diagram of the charging and discharging currents.